Formation and evolution of small planets and moons

我们的研究旨在帮助理解较小的类地行星核心（例如，水星、月球、火星和木卫三）以及外太阳系的冰冷卫星和较小天体的形成和演化。我们将通过以下方式进行研究：(i)通过研究核心结晶过程（项目1），（ii）通过在相关压力和温度条件下确定核心形成材料的物理性质（项目2），以及（iii）通过研究冰冷材料在高压和极低温下的物理性质（包括其振动谱）（项目3）。近期和未来任务的关键观测数据是表面形态、光谱观测、磁场、重力数据，以及月球和火星（通过当前的洞察号任务）的地震观测。磁场的产生依赖于能量来源，包括重力场和结晶潜热的释放。目前，对小型行星核心中这些能量来源的详细了解尚不确定，因为我们甚至不知道它们是自下而上还是自上而下结晶的，这对发电机驱动机制具有一级含义；这些是项目1要解决的主要问题。该项目的主要成果将是从头开始预测与水星、木卫三、火星和月球核心相关的整个压力-温度范围：1)铁合金的绝热温度梯度和熔化曲线，这将使我们能够确定核心结晶模式；2)液体密度和液-固密度对比，这有助于引力能的释放；3)铁合金系统的融合潜热。项目2主要是实验性的，解决了我们在较小的行星核心条件下对铁合金和相关材料的基本物理特性的了解的主要空白。如果我们要了解行星的演化，行星内部的地球物理模型——其结果可以用航天器和地面观测的数据进行测试——是必不可少的，但这些地球物理模型只有在形成核心的材料（主要是铁和铁合金）的物理性质在相关压力和温度范围内被准确地知道的情况下才可靠。目前，在月球、水星、火星和木卫三的内部，通常情况并非如此。令人惊讶的是，即使是最简单的近似——纯铁的核心——目前可用的数据也不足。该项目的主要成果将是在高达~40 GPa的压力下确定铁合金的相图和物理性能，特别是三元体系，如Fe-Ni-Si和Fe-S-Si。项目3解决了另一类材料——水冰和水合硫酸盐盐——它们构成了太阳系外冰体的地幔。对它们施加压力很容易引起晶体结构的变化，在水合硫酸盐的情况下，可以导致水的排出并转变为较低的水合状态。这些结构变化的结果是在冰体内产生分层结构。此外，伴随脱水的大体积变化可能导致地表的整体膨胀和裂陷和/或整体收缩和皱缩。本项目的主要成果包括这些冰质材料在相应压力和温度条件下的相图、状态方程和热导率。除此之外，我们还将确定这些不同相位的振动谱，以便与任务数据进行直接比较。

Our research aims to aid understanding of the formation and evolution of smaller terrestrial planetary cores (e.g., Mercury, the Moon, Mars and Ganymede) and the icy moons and smaller bodies of the outer solar system. We shall do this in the following ways: (i) by investigating core crystallisation processes (Project 1), (ii) by determining the physical properties of core-forming materials under the relevant conditions of pressure and temperature (Project 2) and (iii) by investigating the physical properties of icy materials (including their vibrational spectra) at high pressures and very low temperatures (Project 3).Key observables from recent and future missions are surface morphologies, spectroscopic observations, magnetic fields, gravity data and, for the Moon and for Mars (via the current InSight mission) seismic observations. The generation of magnetic fields relies on energy sources which include the release of gravitational energy and latent heat of crystallisation. Detailed understanding of these energy sources in small planetary cores is currently uncertain, as it is not even known whether they crystallise from the bottom up or from the top down, and this has first order implications for dynamo driving mechanisms; these are the main issues to be addressed by Project 1. Key deliverables of this project will be ab initio predictions over the entire pressure-temperature range relevant to the cores of Mercury, Ganymede, Mars and the Moon of: i) the adiabatic temperature gradients and melting curves of iron-alloys, which will allow us to determine the mode of core crystallisation, ii) the densities of the liquids and the liquid-solid density contrast, which contributes to gravitational energy release, and iii) the latent heat of fusion in iron alloy systems.Project 2 is predominantly experimental, and addresses major gaps in our knowledge of the fundamental physical properties of iron alloys and related materials at the conditions found within the smaller planetary cores. If we are to understand planetary evolution, geophysical modelling of planetary interiors - the outputs of which can be tested against data from spacecraft and ground-based observations - is essential, but these geophysical models will be reliable only if the physical properties of the constituent core-forming materials (essentially iron and iron alloys) are accurately known in the relevant pressure and temperature ranges. At present, this is often not the case for the conditions expected in the interiors of the Moon, Mercury, Mars and Ganymede. Surprisingly, even for the simplest approximation - a core of pure iron - the data currently available are inadequate. Key deliverables of the project will be the determination of the phase diagrams and physical properties of iron alloys, especially ternary systems such as Fe-Ni-Si and Fe-S-Si, at pressures up to ~40 GPa. Project 3 addresses a different class of materials - water ice and hydrated sulphate salts - which form the mantles of the icy bodies of the outer solar system. Application of pressure to these readily induces changes in crystal structure and, in the case of the hydrated sulphate salts, can lead to expulsion of water and transformation to a lower hydration state. The consequence of these structural changes is to produce a layered structure within the icy body. In addition, the large volume changes accompanying dehydration may result in global expansion and rifting and/or global contraction and crumpling of the surface. Key deliverables of this project include the phase diagrams, equations of state and thermal and electrical conductivities of these icy materials under the relevant conditions of pressure and temperature. In addition to this we shall determine the vibrational spectra of these various phases which will allow direct comparison with mission data.